Fortran: [PR93727] Add EX format READ (read_ex)

Implement read_ex in libgfortran to handle EX edit-descriptor input
per Fortran 2023 13.7.2.3.6.  The input field may contain:

  * a hexadecimal-significand form (0X<sig>P<exp>) passed directly
    to the C strtod/strtold family for exact bit-for-bit conversion;
  * any form acceptable for Fw.d input (decimal fallback), including
    INF and NAN representations.

For decimal input without a decimal point the d field of the EX.w.d
descriptor adjusts the exponent exactly as for Fw.d.  BN/BZ blank
handling and the kP scale factor are also supported via the shared
decimal path.

Assisted by: Claude Sonnet 4.6

	PR fortran/93727

libgfortran/ChangeLog:

	* io/read.c (read_ex): New function implementing EX format read.
	* io/io.h (read_ex): Declare.
	* io/transfer.c (formatted_transfer_scalar_read): Add FMT_EX case
	dispatching to read_ex.

gcc/testsuite/ChangeLog:

	* gfortran.dg/EXformat_4.F90: New test covering EX format read
	for kind=4 and 8 (always) and kind=10, 16 (when available):
	hex-significand literals, lowercase prefix, negative binary
	exponent, decimal fallback, d-field adjustment, INF/NAN, zero
	field, and round-trips through write_ex.